This invention relates to a double-layer fabric for use as a screen in the sheet forming and drying sections of a papermachine. In particular, it relates to a double-layer fabric which includes two layers of transversely extending weft wires and longitudinally extending warp wires at least part of which are interwoven with both layers of the weft wires.
Synthetic fabrics for use in papermachines have been in a state of constant development ever since such fabrics were first proposed as replacements for metal screens. Synthetic fabrics are used in the sheet forming section of papermachines and are also useable in the drying section of such machines. Initial fabrics were of single-layer weaves, but later fabrics incorporated double-layer weaves to achieve better stability and retention. However, even with the double-layer weaves, marking of the paper web and high abrasion were found to occur, especially when the fabric was used in the wet end of the sheet forming section of the papermachine.
German OS No. 2,263,476 discloses a monoplanar fabric in which the warp and weft knuckles are disposed in one plane after setting. In the disclosed fabric, the warp wire passes at least twice over a crossing wire of the top weft layer. However, with this fabric, due to the groove-like recesses in diagonal direction between the groups of warp and weft knuckles, marking was still very considerable and could not be eliminated even by grinding on the paper side. Moreover, the fact that the warp bend extended below two weft pairs on the running side made the fabric subject to very rapid wear.
In order to avoid these shortcomings, German OS No. 2,540,490 proposes to pass the warp wire under only a single weft wire on the running side. This enables the free weft bend to become longer, and, at high setting tension, the warp wire can be embedded deeper into the fabric interior. However, the increased life realizable with this technique is limited, since only relatively thin weft wires can be interwoven on the running side. If other than thin weft wires were used, the warp wire could not sufficiently deform the thicker and stiffer weft wire and consequently it would remain on the external side of the fabric.
German OS No. 2,706,235 and Swedish AS No. 7,702,520-3 also deal with double-layer papermachine fabrics and disclose passing the warp wires around only a single weft wire on the paper side. The resultant fabric has a symmetrical weave pattern on the paper and running sides and exhibits high permeability. The long weft bends leave strong marks, so that the screen cannot be used to manufacture paper susceptible to marking.
While, in the above-mentioned papermachine screens, the paper side of the fabric is formed mainly by the weft wires, German Utility Model No. 7,630,822 discloses forming the paper side predominantly from warp wires. In this fabric, the warp wires are disposed above the weft wires in the top layer. Since the paper pulp is likewise oriented in longitudinal direction, resultant problems in sheet removal can be encountered with this fabric.
It is therefore an object of the present invention to develop a double-layer fabric for use as papermachine screen which can be used for the manufacture of paper susceptible to marking and which exhibits good retention and has a long lifetime.